The surface tension σ indicates what work has to be effected in order to increase by a specific amount a surface at the liquid-gas interface. It therefore gives information e.g., on the concentration and effectiveness of surfactants in liquids, e.g. for the quality control of inks or waters in washing and cleaning processes.
With the bubble pressure principle a gas or gas mixture, usually air, is forced through a capillary tube connected to a pneumatic system into a liquid to be analyzed and the internal pressure p of the bubble forming on the capillary tube is measured.
In the maximum bubble pressure method the maximum bubble pressure pmax is measured. The hydrostatic pressure ph acting on the bubble is calculated from the immersion depth hE of the capillary tube, which has to be detected and adjusted in a complicated manner, and the liquid density. The surface tension σ is then calculated with the radius of the capillary rcap in a first approximation according to:σ=rcap/2(pmax−ph)  (1)
In a differential pressure method on a capillary tube derived from this (DE 197 55 291 C1, DE 203 18 463 U1) the dynamic surface tension σ is calculated using the correlation K between the surface tension σ and the differential pressure Δp between the maximum internal pressure pmax and the minimum internal pressure pmin of the bubble:σ=K·Δp with Δp=pmax−pmin  (2)
On the basis of the same action of the hydrostatic pressure on pmin and pmax, unlike in the maximum bubble pressure method, the measurement remains independent of the capillary tube immersion depth.
In surfactant-containing liquids the measured value of the surface tension σ is dependent on the age of the expanding surface, because with increasing bubble life surfactants can be increasingly attached to a newly formed bubble surface. Thus, the bubble pressure principle consequently determines a dynamic surface tension, so that a measured value must always be given in connection with the associated bubble formation time or bubble life tlife, this being understood to mean hereinafter the time between the pressure minimum and pressure maximum of the bubble.
Known bubble pressure methods measure at a clearly defined bubble frequency or bubble life of the exiting gas bubbles, which must be constantly readjusted in accordance with the dynamically changing surface tension (DE 197 55 291 C1), the maximum bubble pressure or the differential pressure at a capillary tube. A controllable air pump or an air flow-controlling valve is required for this.
To be able to sufficiently accurately measure the surface tension, the pressure sensor used must have a high measuring accuracy compared with other applications.
Pressure sensors meeting these demands must be temperature-compensated and calibrated and therefore constitute the most costly component of a measuring system.
Alternatives to the transformation of the bubble pressure into an electrical signal are sound pressure transducers such as condenser, moving coil, crystal and carbon microphones as well as piezoelectric disks (EP 760 472 B1, EP 902 887 A1). Thus, according to EP 760 472 B1 using a cost effective sound pressure transducer the first derivation of the pressure signal after time is measured and by subsequent integration the bubble pressure and from this the surface tension is determined. It is impossible to avoid measurement errors resulting from the influence of the ambient temperature, atmospheric humidity, frequency dependence of the microphones in the transmission behaviour and drift during a measurement. Sound pressure transducers do not meet the accuracy requirements in connection with a pressure measurement without taking additional measures.
It is known from EP 682 588 A1, that in the case of adequately constant air flows the measured bubble frequency of the bubbles forming on a capillary tube is correlated with the surface tension. With decreasing surface tension the bubble frequency rises. The reciprocal of the bubble frequency, the bubble period time, is formed from the bubble life and the so-called bubble dead time (DE 203 18 463 U1). The bubble dead time designates the time between the pressure maximum following the passage of which the bubble collapses and is inflated and bubble detachment. Even minor flow patterns in the liquid or mechanical vibrations influence the bubble detachment in a random manner and consequently lead to high measurement errors on measuring the surface tension through the bubble frequency. The resulting measurement accuracy is not adequate e.g. for the determination of the detergent concentration in the textile cleaning sector.
Hitherto in the textile and dishwashing sector, particularly in the domestic field, no economic, marketable solution is known with which the surface tension can be sufficiently precisely measured for concentration determination of the detergent or washing agent and on the basis of this an automatic dosing or metering.